How to make an animation smoother?
Short Answer
The pacing for an animation is controlled by the proximity of the keyframes over time. In the Dopesheet or the Graph Editor windows you can select keframes and move them horizontally. If the keyframes are moved closer to each other the movement will happen faster. If the Keyframes are moved away from each other, then the movement will happen in more frames and will take longer. Additionally you can use graph editor to adjust the animation curves which will affect the pacing of motion.
Long Answer
The hardflip is quite a complex movement: not all motions occur with the same timing, and the number of keyframes needed to describe it is not the same for all axes, and is different for location, rotation and tilt.
The location movement is quite simple for the X axis: the board keeps travelling forward even when it is in the air, but at a lower speed, then lands and picks up a bit of speed again.
Heightwise the motion is almost a parabola, but there is a sharp movement down once the feet touch the board pushing it down back to the ground.

The rotation is the complex part:
On the X and Z the movement is 360 on X and -180 on Y, and they both start almost immediately after the board is kicked on the back to raise it, but the rotation ends while the board is still in the air and the feet of the skater stabilize it and push it down.These rotations can be described with only two keyframes per channel and are basic bezier curves.

On the Y axis it is a very different story because the movement is sinusoidal to describe the hard initial kick and the landing with the back of the board first and then landing the front last.

So, going back to the original question... Once you have the keyframes you need, then you can control the pacing for the animation by moving the keyframes closer or further away from each other...

Additionally you can fine tune movement by playing with the control points and its handles on the curves on the Graph editor window.

Here's the file used for this example: